Move unittest from lib to pkg.

tests/lib/unittest/mock_stepwise_negative_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #library('unittestTest');
-#import('../../../lib/unittest/unittest.dart');
+#import('../../../pkg/unittest/unittest.dart');
 
 main() {
   test('Mocking: stepwiseValidate', () {
     LogEntryList logList = new LogEntryList('test');
     for (var i = 0; i < 10; i++) {
       LogEntry e = new LogEntry(null, 'foo', [i], Action.IGNORE);
       logList.add(e);
     }
     int total = 0;
     logList.stepwiseValidate((log, pos) {
         return 0;
     });
   });
 }